1. Field of the Invention
The present invention relates to a silicon-on-insulator (SOI) substrate, a method for producing the substrate, a solid-state image pickup device, a method for producing the device, and an image pickup apparatus.
2. Description of the Related Art
Silicon on insulator (SOI) substrates have recently been receiving attention in the field of image sensors as well as high-density CMOS elements and high-withstand-voltage elements because a significant increase in sensitivity should be obtained. For example, SOI substrates are used in whole-area-open-type CMOS image sensors. It is desired to have a higher gettering ability.
Specifically, a SOI substrate has a three-layer structure in which a single-crystal silicon layer used as a device formation region is arranged on a support substrate with a buried insulating film such as a silicon oxide (SiO2) film. In some cases, an epitaxial grown layer is arranged on the single-crystal silicon layer.
SOI substrates have characteristics such as low parasitic capacitance and high radiation hardness and thus should have advantages, such as a higher speed, lower power consumption, latch-up prevention. Hence, SOI substrates are widely used as substrates for high-performance semiconductor elements.
Recently, also in the field of image sensors, SOI substrates have recently been widely used in whole-area-open-type CMOS image sensor structures, in which a significant increase in sensitivity should be obtained, because a photodiode layer can be formed while its thickness is accurately controlled.
Various production processes of SOI substrates are developed. SOI substrates formed by SIMOX in which oxygen is highly doped by ion implantation and SOI substrates formed by bonding are widely commercially used.
In particular, SOI substrates formed by bonding are often used.
The production process is as follows: Two mirror-polished single-crystal silicon substrates are prepared. One is a single-crystal silicon substrate (Substrate A) to be formed into a SOI layer. The other is a single-crystal silicon substrate (Substrate B) that will serve as a support substrate. An oxide film is formed on a surface of at least one of the single-crystal silicon substrates. These single-crystal silicon substrates are bonded to each other in such a manner that the oxide film is sandwiched therebetween. Heat treatment is then performed to increase bonding strength. The thickness of Substrate A is reduced from its backside, affording a SOI substrate.
Processes for reducing the thickness of Substrate A include (a) a process in which Substrate A is subjected to grinding or polishing to a target thickness, (b) a process in which a difference in etching rate due to different impurity concentrations is utilized, (c) a process (an ion-implantation exfoliation process, e.g., the Smart Cut process) including the steps of ion-implanting hydrogen or helium to form a split layer before bonding Substrate A and Substrate B and subjecting the bonded substrates to heat treatment for exfoliation at a temperature lower than a bonding temperature to separate Substrate A at the split layer.
SOI substrates have advantages that good electrical characteristics are achieved and that a uniform silicon layer can be formed. However, structures of SOI substrates are disadvantageous for metal contamination. That is, with respect to most of metal impurities, their diffusion coefficients in silicon oxide film are smaller than those in silicon. Furthermore, metal oxides are stable. In the case where a metal contaminant enters a single-crystal silicon layer from the side of its surface, thus, the metal contaminant does not readily pass through the silicon oxide layer, so that the metal impurity is accumulated in the thin single-crystal silicon layer. Hence, in many cases, SOI substrates are susceptible to metal contamination compared with silicon substrates that do not have SOI structures. This is a severe problem for, in particular, image sensors that are susceptible to luminous-dot defects and dark current due to metal impurities.
SOI substrates, therefore, preferably have a high ability to trap metal impurities and remove them from a single-crystal silicon layer to be formed into an active layer of a semiconductor element, i.e., gettering ability.
Examples of a gettering technique for a SOI substrate include a technique in which a gettering layer 214 is formed (by, for example, ion implantation of a neutral element) on a side of a SOI layer 211 adjacent to a silicon oxide layer 212 in a SOI substrate 210 as shown in FIG. 31A; and a technique in which the gettering layer 214 is formed on a side of the silicon oxide layer 212 adjacent to a support substrate 213 in the SOI substrate 210 as shown in FIG. 31B (for example, see Japanese Unexamined Patent Application Publication No. 2007-318102).
The structure shown in FIG. 31A has a problem in which it is difficult to form an active device region at a deeper position in the SOI layer 211. The structure also has another problem of, for example, the influence of the gettering layer 214 (strain and a dark component due to re-emission of electrons). The structure shown in FIG. 31B has a problem in which the gettering layer 214 is not effective against contamination from the SOI layer 211 side because the gettering layer 214 is located below the silicon oxide layer 212.
In recent years, there have been advances in the reduction in the cell size of image sensors as trends toward miniaturization and an increase in the number of pixels. For example, CCD imagers having a cell size of 1.65 μm have been commercialized. CMOS sensors having a cell size of the order of 1.4 μm have been developed.
The amount of light per pixel is naturally reduced with decreasing pixel size, so that the sensitivity of imagers tends to decrease. A reduction in sensitivity has been prevented by improvements, such as higher light collection efficiency, reductions in reflection and absorption in the upper layer, an increase in the size of a photoelectric conversion region of a bulk (in the depth direction and transverse direction).
However, a cell size of 2 μm or less limits the improvement in light collection efficiency.
A back-illuminated CMOS image sensor as a whole-area-open-type CMOS image sensor has thus been developed (for example, see Japanese Unexamined Patent Application Publication No. 2004-134672). A photoelectric conversion unit is arranged in a single-crystal silicon layer; hence, afterglow and dark characteristics are not impaired. The back-illuminated CMOS image sensor is thus promising.
Methods for producing it include a method in which a SOI substrate (formed by SIMOX, bonding, or the like) is utilized; and a method in which the use of an epitaxially grown substrate having an epitaxially grown layer results in a thin silicon (Si) layer which is a light-incident portion. In particular, a SOI substrate formed by bonding is promising from the viewpoint of achieving good productivity and a good quality of a SOI layer.